Smectite clays are commercially important minerals, bentonite and hectorite in particular. Appropriately processed, smectite clays are excellent viscosifiers, binders, film formers, fabric softeners, and retention aid additives. These clays are 2:1 type layer silicates with an expandable structure. They are highly colloidal and may readily swell in water to form viscous, thixotropic gels which renders the clays useful as viscosity builders in many industries. For instance, they are used to provide rheological control in coatings, cosmetics, drilling muds, greases, suspension aids, agricultural sprays and the like.
By smectite clays we mean natural bentonite and hectorite clays. These natural smectite clays are products that are formed by, for instance, the decomposition of igneous rocks. Therefore, clays are relatively abundant in the environment but their chemical composition may vary from deposit to deposit. Even clay samples taken within one clay deposit can differ from each other. These chemical variations result in for instance, differences in clay layer charge composition and density, impurity content and crystallite sizes. To counteract such variations, some smectite-like clays have been synthesized on an industrial scale. Such synthetic clays are uniform in charge density and are virtually impurity free. As a result, synthetic clays behave very differently from natural clays when used as, for instance, a Theological additive.
A crude natural smectite clay includes both clay and impurities. Such a crude clay can be beneficated or purified so that some or all of the impurities in the crude clay have been removed.
Most smectite clays are sold as fine powders. As with most minerals, however, these powders are difficult and expensive to handle. To provide the user of the clay with a product which is more ready for the user's application, clays have been sold by the manufacturer or distributor as aqueous slurries. Such slurries can be easily stored, shipped, transferred, e.g. pumped and metered, with significantly less capital expenditure and many fewer problems than are associated with dry mineral powders.
Generally, however, it is not economical or practical to ship smectite clays as aqueous slurries because of the large quantity of water normally present in such slurries. This problem arises because smectites are indeed good viscosifiers of water at low loadings; for this reason it has not generally been possible to produce aqueous slurries high in clay solid by the usual methods. In general, the viscosities of the slurries become so high that they cannot readily be pumped by conventional equipment and gelling upon standing becomes a problem. Obviously it is desirable from an economic perspective to produce an aqueous clay slurry high in clay content.
Clay dispersions in water containing around 3 to 5 wt. % clay form viscous gels. The gelling has been prevented in the past by adding a peptizing agent—it is known that metaphosphates, citrates, polyacrylates etc. cause a significant reduction in clay slurry viscosity when added to a clay dispersion. Of these peptizing agents, tetrasodiumpyrophosphate (TSPP) is by far the most effective dispersant in reducing the clay slurry viscosity. However, TSPP hydrolyzes slowly to orthophosphate, which is ineffective in reducing clay slurry viscosity. The viscosity of a clay slurry which is treated with TSPP increases over time and often the slurry becomes then unusable.
Among the prior art that is pertinent to the present invention are the following patents.
U.S. Pat. No. 6,024,790 discloses an alkaline earth bentonite that can be activated by the use of sodium citrate or other organic sequestering agents as an activator. Dry blends of the activator and the bentonite are disclosed.
U.S. Pat. No. 5,779,785, incorporated by reference in its entirety, discloses an aqueous slurry of smectite clay of elevated solids content comprising an aqueous solution or emulsion of a salt of a low molecular weight amine salt and a smectite clay. The amine salt prevents the smectite from swelling appreciably, whereby the slurry can be shipped and stored.
U.S. Pat. No. 5,582,638, incorporated by reference in its entirety, discloses a thickening agent that is based on at least one synthetic phyllosilicate that contains at least one additive from a disclosed group of organic phosphorus compounds, including hydroxyethane-1,1-diphosphonic acid and hydroxyethane disphosphonic acid sodium salt.
U.S. Pat. No. 5,391,228 discloses a method for preparing a time-stable, low-viscosity, pumpable high solids aqueous bentonite slurry. The aqueous solution comprises at least a first and second salt component in which there is dispersed a powdered bentonite. The first salt component is selected to give, if used alone, a low viscosity to the slurry. The second salt component is selected to act in combination with a first salt component to further decrease the viscosity of the specified high solids content or to increase the solids content while maintaining the low viscosity.
U.S. Pat. No. 5,223,098 discloses a bentonite swelling clay provided to a paper making mill as a fluid concentrate containing more than 15% bentonite. Swelling of the bentonite is prevented by an inorganic electrolyte in the concentrate, and the bentonite swells upon dilution either before or after addition of the cellulosic suspension.
U.S. Pat. No. 5,266,538 discloses a smectite clay in an elevated solids aqueous slurry that is composed of smectite clay slurried or dispersed in water containing a non-dispersant salt of a monovalent cation, such as sodium chloride. This patent teaches that a dispersant salt results in high viscosity since the dispersant end of the molecule tends to disperse loose aggregates of smectite.
U.S. Pat. No. 5,151,218 discloses phosphoric acid esters and their salts that can be used as dispersants or dispersing aids in paints and molding compositions.